..his reward is [due] from Allah. [Shuraa 42:40]

And the retribution for an evil act is an evil one like it, but whoever pardons and makes reconciliation –his reward is [due] from Allah. Indeed, He does not like wrongdoers.

What does this mean; that his ajr/reward rests with Allah?

It means; “The pages of the Quran are unable to hold this ajr/reward that is mentioned. That, also the dunya/world and everything within it could not carry this ajr so that it can be mentioned. So Allah (swt) preserved it in Ilm AL-Ghayb [the knowledge of the Unseen] so that he can reward those who forgive/pardon others and make peace with them with an unimaginable reward!